In vivo tumor immunity and in vitro immunologic cross-reactivity between SV-40 virus transformed prostatic tissue and fetal antigens of LVG/LAK strain of hamsters has been studied. The hamsters immunized with fetal atigens demonstrated significant resistance to tumor growth. Humoral cytotoxicity using complement in microtest plates also demonstrated significant cytotoxicity against tumor cells. Immune sera from hamsters injected with fetal tissue and absorbed with SV-40 transformed prostatic tissue had markedly lower cytotoxicity indices than the transformed tissue against unabosrbed immune sera. These experiments have shown in vivo and in vtro cross-reactivity between fetal antigens and tumor associated antigens of transformed prostatic tissue with elevated tartrate inhibited acid phosphatase indicative of epithelial transformation by SV-40 virus.